The present invention relates to an infrared detection device utilizing the spontaneous polarization change caused in a pyroelectric thin film with the temperature rise thereof by the irradiation of infrared rays, in particular, to an infrared image sensing device having a linear or two-dimensional array of a plurality of sensor elements each comprising a pyroelectric thin film.
It is well-known that infrared sensors comprising a crystal of a semiconductor such as HgCdTe or InSb are used as a high sensitivity and high response speed infrared detection device for 4- to 5- micron band and 8- to 12- micron band. These sensors generally have to be operated at the liquid nitrogen temperature or lower. Moreover, these sensors need an optical scanning mechanism in order for obtaining picture elements of infrared images. Accordingly, the infrared image sensing device using these sensors is expensive and comparatively large-sized, and the application thereof is limited to special fields such as medical, military and others.
On the other hand, a low-cost infrared image sensing device suitable for monitoring a human body or machine under operation in a room during night, for instance, is demanded for security purposes. The sensor for these purposes is required to be neither high sensitivity nor the high response speed compared with those required in the above-mentioned special fields.
Infrared sensors comprising a pyroelectric material are being developed, aiming at the application as mentioned above. The pyroelectric infrared sensors are operable in the room temperature, and hence, can comply with the low-cost requirement, while offering a sensitivity and response speed enough for the application.
Pyroelectric material is a kind of ferroelectric material such as PbTiO3 and the spontaneous polarization thereof changes when the temperature of the material is raised by the irradiation of infrared rays. According to the change in the spontaneous polarization, the surface charge of pyroelectric material changes. The change in the surface charge is detected as a voltage or a current. The sensor elements each comprising a pyroelectric material are arranged as a linear or two-dimensional array. The signal output from the sensor elements constitute the picture elements of an infrared image.
Various pyroelectric infrared sensor elements are reported, including 1) a structure comprising a thin plate of pyroelectirc ceramics (R. E. Flannery et al., SPIE Vol. 1689 Infrared Imaging System (1992)/379) and 2) another structure comprising a pyroelectric thin film formed by sputtering a pyroelectric material (R. Takayama et al., National Technical Report Vol. 39 No. 4 August 1993, p. 122).
In addition, although being not limited to the application to the infrared sensors, reported is a method of forming a pyroelectric thin film by using a solution of a precursor of a pyroelectric material (J. Chen et al., J. A. Phys. 71 (9), 1 May 1992, p. 4465; N. Tohge et al. J. Am. Ceram. Soc. 74 (1) 67-71 (1991)).
The above third method is referred to as a sol-gel process. That is, a solution composed of the precursor of a pyroelectric material is coated to form a layer on a surface, and the coated layer is then converted into a pyroelectric thin film by a heat treatment. Since the procedure and equipment necessary for such sol-gel process are relatively simple compared with those for the conventional thin film technology such as sputtering and chemical vapor deposition, the advantage of forming a pyroelectric thin film at a low-cost can be offered.